System for the detection of cardiac events

Abstract

Heart disease is the leading cause of death in the United States. A heart attack (also known as an Acute Myocardial Infarction (AMI)) typically results from a thrombus that obstructs blood flow in one or more coronary arteries. The extent of damage to the myocardium from an AMI is strongly dependent upon the length of time prior to restoration of blood flow to the heart muscle. Acute myocardial infarction and ischemia may be detected from a patient's electrocardiogram (ECG) by noting an ST segment shift (i.e., voltage change) over a relatively short (less than 5 minutes) period of time. The present invention is a guardian system including electrodes, a cardiosaver device having AMI detection capability and an alarm means to warn the patient that they have had an AMI or other serious cardiac event and should immediately seek medical attention. Such a warning would facilitate getting medical treatment quickly to restore blood flow to the patient's heart muscle. The present invention provides accuracy of detection by comparing a recently collected electrogram data with baseline electrogram data collected at an earlier time when the heart signals were normal. False positives are reduced by requiring multiple detections separated by a predetermined time interval. The present invention can identify and differentiate between an acute myocardial infarction, exercise induced ischemia and other anomalous cardiac conditions. In the preferred embodiment of the present invention the electrodes and the cardiosaver are implanted and the guardian system includes an external alarm system that can produce an external alarm signal and also has means to turn off the alarm.

What is claimed is:  
     
       1. A system for detecting a cardiac event in a human patient the system including: 
       at least two electrodes implanted in the patient for obtaining the electrical signal from the patient's heart, the electrical signal being an electrogram;  
       an implanted cardiosaver including  
       (a) analog-to-digital converter circuitry for digitizing the electrogram to produce electrogram segments having a time duration that is at least 1 second;  
       (b) means for processing a first electrogram segment at a first predetermined time to extract at least one baseline heart signal parameter of the patient;  
       (c) memory means for storing the at least one baseline heart signal parameter;  
       (d) means for processing a second electrogram segment at a later, second predetermined time to extract at least one heart signal parameter of the patient; and,  
       (e) processor means coupled to the memory means designed to detect the cardiac event when the at least one heart signal parameter extracted at the second predetermined time shifts by more than a predetermined threshold amplitude from the at least one baseline heart signal parameter extracted at the first predetermined time.  
     
     
       2. The system of  claim 1  where the cardiac event is an acute myocardial infarction. 
     
     
       3. The system of  claim 1  where one heart signal parameter is the average voltage of the ST segments of one or more beats of the electrogram segments. 
     
     
       4. The system of  claim 1  where one heart signal parameter is the ST deviation of one or more beats of the electrogram segments. 
     
     
       5. The system of  claim 1  where one heart signal parameter includes the voltage amplitude of the T wave of one or more beats of the electrogram segments. 
     
     
       6. The system of  claim 1  further including memory means within the implanted cardiosaver for storing the first electrogram segment. 
     
     
       7. The system of  claim 1  further including memory means within the implanted cardiosaver for storing the second electrogram segment. 
     
     
       8. The system of  claim 1  further including an alarm sub-system within the implanted cardiosaver, the alarm sub-system having the capability to warn the patient that the cardiac event has occurred. 
     
     
       9. The system of  claim 1  further including two-way wireless communications means between the implanted cardiosaver and an external device. 
     
     
       10. The system of  claim 9  where the external device is an external alarm system capable of producing an audible external alarm signal. 
     
     
       11. The system of  claim 10  where the external alarm system further includes means to turn off the audible external alarm signal. 
     
     
       12. The system of  claim 10  further including an alarm sub-system within the implanted cardiosaver, the alarm sub-system having the capability to warn the patient that the cardiac event has occurred. 
     
     
       13. The system of  claim 1  further including means for extracting one heart signal parameter from a third electrogram segment at a third predetermined time and processor means coupled to the memory means programmed to detect the cardiac event when the one heart signal parameter extracted at both the second and third predetermined times shifts by more than a predetermined threshold amplitude from the same one baseline heart signal parameter extracted a the first predetermined time. 
     
     
       14. The system of  claim 13  further including means for extracting one heart signal parameter from a fourth electrogram segment at a fourth predetermined time and processor means coupled to the memory means programmed to detect the cardiac event when the one heart signal parameter extracted at the second, third and fourth predetermined times shifts by more than a predetermined threshold amplitude from the same one baseline heart signal parameter extracted a the first predetermined time. 
     
     
       15. The system of  claim 14  where the interval between the second predetermined time and the third predetermined time is more than 10 seconds. 
     
     
       16. The system of  claim 1  where at least one of the electrodes is located within the heart. 
     
     
       17. The system of  claim 16  where the electrode located within the heart is located within the right ventricle. 
     
     
       18. The system of  claim 16  where the electrode located within the heart is located within the right atrium. 
     
     
       19. The system of  claim 1  where at least one of the electrodes is located outside of the heart. 
     
     
       20. The system of  claim 1  where at least one electrode is located subcutaneously. 
     
     
       21. The system of  claim 20  where at least one subcutaneously located electrode is located on the patient's left side. 
     
     
       22. The system of  claim 1  where the cardiac event is ischemia occurring at an elevated heart rate, the elevated heart rate is a heart rate being greater than a predetermined rate that is programmed into the implanted cardiosaver. 
     
     
       23. The system of  claim 1  where the electrogram segments are less than 30 seconds long. 
     
     
       24. The system of  claim 1  where the electrogram segments are more than 4 seconds long. 
     
     
       25. The system of  claim 1  where the interval between the first predetermined time and the second predetermined time is between 20 and 28 hours. 
     
     
       26. The system of  claim 1  where the cardiac event is a very rapid heart rate, the very rapid heart rate is a heart rate being greater than a predetermined rate that is programmed into the implanted cardiosaver. 
     
     
       27. The system of  claim 1  where the cardiac event is a very slow heart rate, the very slow heart rate is a heart rate being less than a predetermined rate that is programmed into the implanted cardiosaver. 
     
     
       28. The system of  claim 1  where the cardiac event is an unsteady heart rate. 
     
     
       29. The system of  claim 1  where a cardiac event is detected only if the shift by more than a selective threshold amplitude of the at least one heart signal parameter occurs in a majority of the beats of the second electrogram segment processed at the second predetermined time. 
     
     
       30. The system of  claim 1  further including means to exclude from processing any baseline electrogram segment that is not indicative of the patient's normal heart signal. 
     
     
       31. The system of  claim 30  where the normal heart signal has an average heart rate that is less than a preset limit. 
     
     
       32. The system of  claim 30  where the normal heart signal within the baseline electrogram segment has no more than two shortened beats having an R—R interval less than a preset limit. 
     
     
       33. The system of  claim 30  where the normal heart signal has an average R—R interval that is less than a preset limit. 
     
     
       34. The system of  claim 30  where the normal heart signal has an average R—R interval that is greater than a preset limit. 
     
     
       35. The system of  claim 1  where one baseline heart signal parameter is the average value of a per-beat heart signal parameter for at least two beats of the first electrogram segment at the first predetermined time. 
     
     
       36. The system of  claim 1  where one baseline heart signal parameter is the average value of a per-beat heart signal parameter for at least two normal beats of the first electrogram segment at the first predetermined time. 
     
     
       37. The system of  claim 36  where a normal beat is determined by having an R—R interval that is less than a preset time period. 
     
     
       38. The system of  claim 36  where a normal beat is determined by having an R—R interval that is greater than a preset time period. 
     
     
       39. The system of  claim 36  where a normal beat is determined by having an ST deviation that is less than a preset threshold. 
     
     
       40. A system for identifying a cardiac event in a human patient including: 
       at least two electrodes implanted in the patient for obtaining the electrical signal from the patient's heart, the electrical signal being an electrogram;  
       an implanted cardiosaver including  
       (a) analog-to-digital converter circuitry for digitizing the electrogram to produce electrogram segments having a time duration that is at least 1 second;  
       (b) memory means designed to store a baseline electrogram segment at a first predetermined time;  
       (c) memory means designed to store a recently collected electrogram segment at a second predetermined time at a time that is later than the first predetermined time;  
       (d) processor means coupled to said memory means for comparing the recently collected electrogram segment stored in the memory means at the second predetermined time with the baseline electrogram segment stored in the memory means at the first predetermined time; and,  
       (e) means for identifying that the cardiac event has occurred when ST deviation of at least one beat of the recently collected electrogram segment shifts by more than a predetermined amplitude from the average ST deviation of at least two beats of the baseline electrogram segment.  
     
     
       41. The system of  claim 40  where the cardiac event is an acute myocardial infarction. 
     
     
       42. The system of  claim 40  further including an alarm sub-system within the implanted cardiosaver with the capability to warn the patient that a cardiac event has occurred. 
     
     
       43. The system of  claim 42  where the alarm sub-system includes produces an internal alarm signal that is selected from the group consisting of a mechanical vibration, an audible sound or a subcutaneous electrical tickle. 
     
     
       44. The system of  claim 40  further including memory means for storage of electrogram segments at one or more additional predetermined times and processor means for comparing the electrogram segments at the additional predetermined times with the baseline electrogram segment. 
     
     
       45. The system of  claim 40  where at least one of the electrodes is located within the heart. 
     
     
       46. The system of  claim 45  where the at least one electrode located within the heart is located within the right ventricle. 
     
     
       47. The system of  claim 45  where the at least one electrode located within the heart is located within the right atrium. 
     
     
       48. The system of  claim 40  where at least one of the electrodes is located outside of the heart. 
     
     
       49. The system of  claim 48  where at least one electrode is located in the superior vena cava. 
     
     
       50. The system of  claim 40  where at least one of the electrodes is located subcutaneously. 
     
     
       51. The system of  claim 50  where a subcutaneously located electrode is located on the patient's left side. 
     
     
       52. The system of  claim 40  where the electrogram segments are less than 30 seconds long. 
     
     
       53. The system of  claim 40  where the electrogram segments are more than 4 seconds long. 
     
     
       54. The system of  claim 40  where the interval between the first predetermined time and the second predetermined time is between 20 and 28 hours. 
     
     
       55. The system of  claim 40  where the cardiac event is ischemia occurring at an elevated heart rate, the elevated heart rate being greater than a predetermined rate that is programmed into the implanted cardiosaver. 
     
     
       56. The system of  claim 40  where a cardiac event is detected when the ST deviation of a majority of the beats of the recently collected electrogram segment shift by more than a predetermined amplitude from the average ST deviation of at least two beats of the baseline electrogram segment. 
     
     
       57. The system of  claim 40  where a cardiac event is detected when the ST deviation of at least two thirds of the beats of the recently collected electrogram segment shift by more than a predetermined amplitude from the average ST deviation of at least two beats of the baseline electrogram segment. 
     
     
       58. The system of  claim 40  where a cardiac event is detected when the ST deviation of at least 6 out of 8 beats of the recently collected electrogram segment shift by more than a predetermined amplitude from the average ST deviation of at least two beats of the baseline electrogram segment. 
     
     
       59. The system of  claim 40  further including means for calculating the R—R interval between successive beats of each electrogram segment. 
     
     
       60. The system of  claim 40  further including means to exclude from processing any baseline electrogram segment that is not indicative of the patient's normal heart signal. 
     
     
       61. The system of  claim 60  where the normal heart signal has an average heart rate that is less than a preset limit. 
     
     
       62. The system of  claim 60  where the normal heart signal within the baseline electrogram segment has no more than two shortened beats having an R—R interval less than a preset limit. 
     
     
       63. The system of  claim 60  where the normal heart signal has an average R—R interval that is less than a preset limit. 
     
     
       64. The system of  claim 60  where the normal heart signal has an average R—R interval that is greater than a preset limit. 
     
     
       65. A system for notifying a patient that a cardiac event has been detected, the system including: 
       an implanted device designed to detect the cardiac event;  
       alarm means that actuates an internal alarm signal from the implanted device to notify the patient when the implanted device detects the cardiac event, and  
       an external alarm system adapted for operation by the patient, the external alarm system having two-way wireless communication to and from the implanted device, and also having an external alarm signal that is actuated when the implanted device detects the cardiac event, the external alarm system having patient operated means for turning off both the internal alarm signal in the implanted device and the external alarm signal being emitted by the external alarm system at some time, as determined by the patient, after the implanted device detects a cardiac event.  
     
     
       66. The system of  claim 65  where the internal alarm signal is selected from the group consisting of a mechanical vibration, an audible sound or a subcutaneous electrical tickle. 
     
     
       67. The system of  claim 65  further including timing means to turn off the internal alarm signal after a preset time. 
     
     
       68. The system of  claim 65  where the cardiac event is a heart attack. 
     
     
       69. The system of  claim 68  where the implanted device is designed to process electrical signals from the patient's heart and the heart attack is detected by identification of an excessive ST shift in the electrical signals from the patient's heart. 
     
     
       70. The system of  claim 65  where the cardiac event is an arrhythmia. 
     
     
       71. The system of  claim 70  where the arrhythmia is selected from the group consisting of tachycardia, bradycardia, unsteady heart rate, bigeminal rhythm, premature ventricular contractions, premature atrial contractions and atrial fibrillation. 
     
     
       72. The system of  claim 65  where the cardiac event is exercise induced ischemia. 
     
     
       73. The system of  claim 65  where the external alarm system further includes means to verify the operational status of the implanted device. 
     
     
       74. The system of  claim 65  where the implanted device actuates an external alarm signal produced by the external alarm system when a cardiac event is detected, the external alarm system also including means to turn off the external alarm signal. 
     
     
       75. The system of  claim 74  further including timing means to turn off the external alarm signal after a preset time. 
     
     
       76. The system of  claim 65  where there are at least two different internal alarm signals. 
     
     
       77. The system of  claim 76  where at least one of the internal alarm signals is “See Doctor” alarm indicating that the patient should contact his doctor. 
     
     
       78. The system of  claim 76  where at least one of the internal alarm signals is a patient warning to immediately seek medical attention. 
     
     
       79. The system of  claim 65  where the external alarm system also includes means to provide a warning message to the patient by a text display. 
     
     
       80. The system of  claim 65  where the external alarm system includes means to provide a warning message to the patient by a speech message. 
     
     
       81. A system for detecting a cardiac event from the electrogram of a patient's heart as measured by implantable electrodes, the system including: 
       (a) electrical circuitry means for determining the time of occurrence of an R wave of the electrogram;  
       (b) processor means for calculating the time period between successive R waves, that time period being called an R—R interval;  
       (c) processor means for calculating the average signal amplitude of at least one sub-segment of the electrogram occurring within the R—R interval, the sub-segment having a start time and a time duration;  
       (d) means for adjusting the start time and/or time duration of the sub-segment responsive to the calculated R—R interval; and,  
       (e) processing means for detecting that the cardiac event has occurred by comparing the average signal amplitude of at least one sub-segment of at least one beat of the electrogram with a predetermined threshold amplitude.  
     
     
       82. The system of  claim 81  where the cardiac event is an acute myocardial infarction. 
     
     
       83. The system of  claim 81  where the cardiac event is exercise induced ischemia. 
     
     
       84. The system of  claim 81  where the sub-segment of the electrogram includes at least a portion of the ST segment of the electrogram. 
     
     
       85. The system of  claim 81  where the sub-segment of the electrogram includes at least a portion of the PQ segment of the electrogram. 
     
     
       86. The system of  claim 81  where the sub-segment of the electrogram includes the peak of the T wave. 
     
     
       87. The system of  claim 81  further including means to exclude any beat where the R—R interval is shorter than a predetermined time period from the processing means for detecting a cardiac event. 
     
     
       88. The system of  claim 87  where the predetermined time period is less than 60 milliseconds. 
     
     
       89. A system for detecting a cardiac event from the electrogram of a patient's heart as measured by implantable and/or skin surface electrodes, the system including: 
       (a) electrical circuitry means for determining the time of occurrence of an R wave of the electrogram;  
       (b) processor means for calculating the time period between successive R waves, that time period being called an R—R interval;  
       (c) processor means for calculating the average signal amplitude of the ST segment and the average signal amplitude of the PQ segment of the electrogram occurring within the R—R interval, the sub-segment having a start time and a time duration;  
       (d) means for adjusting the start time and/or time duration of the ST segment responsive to the calculated R—R interval;  
       (e) means for calculating an ST deviation by subtracting the average signal amplitude of the PQ segment from the average signal amplitude of the ST segment; and,  
       (f) processing means for detecting that the cardiac event has occurred by comparing the amplitude of the ST deviation of at least one beat of the electrogram with a predetermined threshold amplitude.  
     
     
       90. The system of  claim 89  where the processing means detects that a cardiac event has occurred when the amplitude of the ST deviation measured at a second time exceeds the amplitude of a baseline ST deviation at a first time by an amount that is greater than the predetermined threshold amplitude. 
     
     
       91. The system of  claim 90  where the first time occurs at least 30 minutes before the second time. 
     
     
       92. The system of  claim 90  where the first time occurs between 23 and 25 hours before the second time. 
     
     
       93. A method for detecting that a cardiac event has occurred, the method including the following steps: 
       (a) implanting at least two electrodes within a human subject at locations designed to pick up an electrical signal from a patient's heart, the electrical signal being an electrogram;  
       (b) connecting the at least two electrodes to an implanted cardiosaver device that is designed to digitize the electrogram, the cardiosaver device also having electrical circuitry means for creating electrogram segments having a duration of at least 1 second;  
       (c) using a physician's programmer to program the cardiosaver to process and store in memory baseline electrogram segments at specified times throughout each and every day;  
       (d) programming specific times when the cardiosaver will collect, process and place in memory recent electrogram segments that occur after the baseline electrogram segment has been placed in memory;  
       (e) programming the cardiosaver to detect that the cardiac event has occurred when the amplitude of the ST deviation of at least one beat of a recent electrogram segment shifts by more than a predetermined amplitude as compared with the average ST deviation of at least two beats of the baseline electrogram segment.  
     
     
       94. The method of  claim 93  where the specified times for baseline electrogram segment processing are preset within the cardiosaver and do not need to be programmed. 
     
     
       95. The method of  claim 93  where the specified times for processing and storing baseline electrogram segments are at a set time interval. 
     
     
       96. The method of  claim 95  where the set time interval is between 30 minutes and 90 minutes. 
     
     
       97. The method of  claim 95  where the set time interval is approximately 60 minutes. 
     
     
       98. The method of  claim 93  where the interval at which the cardiosaver will process recent electrogram segments is preset within the cardiosaver and does not need to be programmed. 
     
     
       99. The method of  claim 98  where the preset time interval for processing recent electrogram segments is less than approximately 2 minutes. 
     
     
       100. The method of  claim 98  where the preset time interval for processing recent electrogram segments is approximately 30 seconds. 
     
     
       101. The method of  claim 93  where the programming in step e further includes specification of the required number of beats of the recent electrogram that must shift by more than the predetermined amplitude to detect the cardiac event. 
     
     
       102. The method of  claim 93  where at least one electrode lies within the heart of the patient. 
     
     
       103. The method of  claim 93  further including a step (f) of programming the cardiosaver to generate an alarm when the cardiac event is detected. 
     
     
       104. The method of  claim 93  where the cardiosaver is designed to generate an alarm signal when the cardiac event is detected. 
     
     
       105. The method of  claim 93  where the cardiac event is an acute myocardial infarction. 
     
     
       106. The method of  claim 93  where the cardiac event is exercise induced ischemia. 
     
     
       107. The method of  claim 93  further including a step (f) of programming a high heart rate limit into the cardiosaver. 
     
     
       108. The method of  claim 93  further including a step (f) of programming a low heart rate limit into the cardiosaver. 
     
     
       109. The method of  claim 93  further including a step (f) of programming an elevated heart rate limit into the cardiosaver, the cardiac event being an acute myocardial infarction if the patient's heart rate is below the elevated heart rate limit and the cardiac event being exercise induced ischemia if the patient's heart rate is above the elevated heart rate limit. 
     
     
       110. A system for the detection of a cardiac event of a human patient, the system including: 
       (a) at least two electrodes implanted in the patient for obtaining an electrical signal from the patient's heart, the electrical signal being an electrogram;  
       (b) analog-to-digital converter circuitry for digitizing the electrogram to produce electrogram segments having a time duration that is at least 1 second;  
       (c) means for determining the time of occurrence of an R wave for each beat within an electrogram segment produced by the analog-to-digital conversion circuitry;  
       (d) means for calculating an average heart rate of the electrogram segment;  
       (e) processor means designed to detect that the cardiac event has occurred, the criteria for detection being dependent upon the average heart rate of the electrogram segment.  
     
     
       111. The system of  claim 110  further including means for classifying the average heart rate as being in one of at least two different heart rate ranges. 
     
     
       112. The system of  claim 111  where the criteria for detecting that the cardiac event has occurred is different for each one of the at least two different heart rate ranges. 
     
     
       113. The system of  claim 111  where the at least two different heart rate ranges include a normal range and at least one elevated range. 
     
     
       114. The system of  claim 113  where the cardiac event is and acute myocardial infarction if the average heart rate of the electrogram segment is in the normal range. 
     
     
       115. The system of  claim 113  where the cardiac event is exercise induced ischemia if the average heart rate of the electrogram segment is in the at least one elevated range. 
     
     
       116. The system of  claim 113  where there are at least two elevated ranges, each range having different detection criteria for detecting the cardiac event. 
     
     
       117. The system of  claim 113  further including means for determining if the average heart rate of the electrogram segment is below a low heart rate limit, the low heart rate limit being a lower bound of the normal range. 
     
     
       118. The system of  claim 110  further including means for determining if the average heart rate of the electrogram segment is higher than a preset high heart rate limit. 
     
     
       119. The system of  claim 118  where the processor means detects a high heart rate cardiac event when the average heart rate of the electrogram segment is greater than the high heart rate limit. 
     
     
       120. A system for detection of a cardiac event of a human patient, the system including an implanted cardiosaver designed to detect the cardiac event and an external alarm system, the external alarm system including an external alarm transceiver and a handheld computer, the external alarm transceiver having a two-way wireless communication means for sending data to and receiving data from the implanted cardiosaver, the external alarm transceiver also having a standardized interface allowing the external alarm transceiver to be inserted into a standardized interface slot in the handheld computer. 
     
     
       121. The system of  claim 120  where the standardized interface is selected from the group consisting of a compact flash adapter interface, a secure digital (SD) adapter interface, a multi-media card interface, a memory stick interface, or a PCMCIA adapter interface. 
     
     
       122. The system of  claim 120  where the handheld computer runs an operating system selected from the group consisting of a version of Microsoft Windows, the Palm operating system or the Linux operating system. 
     
     
       123. The system of  claim 120  where the handheld computer is a Pocket PC. 
     
     
       124. The system of  claim 120  where the cardiosaver is designed to transmit selected data to the external alarm system and the handheld computer is designed to re-transmit the selected data to a geographically remote location through a wireless data transmission means. 
     
     
       125. The system of  claim 124  where the wireless data transmission means uses the internet. 
     
     
       126. The system of  claim 120  where the handheld computer includes an integrated cell phone. 
     
     
       127. The system of  claim 120  where the external alarm transceiver is also designed to be insertable into a laptop PC. 
     
     
       128. The system of  claim 127  where the laptop PC is the physician's programmer for programming the cardiosaver. 
     
     
       129. The system of  claim 120  where the cardiosaver is designed to transmit electrogram segment data to the external alarm system and the handheld computer is designed to display the transmitted electrogram segment data. 
     
     
       130. A system for detecting a cardiac event in a human patient the system including 
       at least two electrodes attached to the patient's skin for obtaining the electrical signal from the patient's heart, the electrical signal being an electrocardiogram;  
       a non-implanted cardiosaver including  
       (a) analog-to-digital converter circuitry for digitizing the electrocardiogram to produce electrocardiogram segments having a time duration that is at least 1 second;  
       (b) means for processing a first electrocardiogram segment at a first predetermined time to extract at least one baseline heart signal parameter of the patient;  
       (c) memory means for storing the at least one baseline heart signal parameter;  
       (d) means for processing a second electrocardiogram segment at a later, second predetermined time to extract at least one heart signal parameter of the patient; and,  
       (e) processor means coupled to the memory means designed to detect the cardiac event when the at least one heart signal parameter extracted at the second predetermined time shifts by more than a predetermined threshold amplitude from the at least one baseline heart signal parameter extracted at the first predetermined time.  
     
     
       131. A system for identifying a cardiac event in a human patient including: 
       at least two electrodes attached to the patient's skin for obtaining the electrical signal from the patient's heart, the electrical signal being an electrocardiogram;  
       a non-implanted cardiosaver including  
       (a) analog-to-digital converter circuitry for digitizing the electrocardiogram to produce electrocardiogram segments having a time duration that is at least 1 second;  
       (b) memory means designed to store a baseline electrocardiogram segment at a first predetermined time;  
       (c) memory means designed to store a recently collected electrocardiogram segment at a second predetermined time at a time that is later than the first predetermined time;  
       (d) processor means coupled to said memory means for comparing the recently collected electrocardiogram segment stored in the memory means at the second predetermined time with the baseline electrocardiogram segment stored in the memory means at the first predetermined time; and,  
       (e) means for identifying that the cardiac event has occurred when ST deviation of at least one beat of the recently collected electrocardiogram segment shifts by more than a predetermined amplitude from the average ST deviation of at least two beats of the baseline electrogram segment.  
     
     
       132. A system for the detection of a cardiac event of a human patient, the system including: 
       (a) at least two electrodes attached to the patient's skin for obtaining an electrical signal from the patient's heart, the electrical signal being an electrocardiogram;  
       (b) analog-to-digital converter circuitry for digitizing the electrocardiogram to produce electrocardiogram segments having a time duration that is at least 1 second;  
       (c) means for determining the time of occurrence of an R wave for each beat within an electrocardiogram segment produced by the analog-to-digital conversion circuitry;  
       (d) means for calculating an average heart rate of the electrocardiogram segment;  
       (e) processor means designed to detect that the cardiac event has occurred, the criteria for detection being dependent upon the average heart rate of the electrocardiogram segment.